It seems like in my Kobo works completely fine when wireless is on (adjusted sleep policy for "always on"). When I try manually turning off the wireless either using airplane mode, or just deselecting the Wifi through the Power Controls widget, the Kobo will crash within a few minutes. THis used to be the safe haven fix in earlier firmware versions, but not the current one.

Unfortunately wi-fi is a bit of a power drain, so that kind of sucks... anyways I hope the kobo developers are working towards getting that fixed!